Conventionally, semiconductor devices having what is called a MOS structure, such as IGBT (Insulated Gate Bipolar Transistor) and MOSFET (MOS Field Effect Transistor), have been known. Such MOS-type semiconductor devices use overvoltage protection diodes made up of series-connected Zener diodes as overvoltage protection measures. More specifically, in the overvoltage protection diodes, n-type semiconductor layers and p-type semiconductor layers are alternately arranged adjacent to each other (e.g., see Patent Literature 1). In the case of IGBT, overvoltage protection diodes are provided between a collector terminal and a gate terminal and a gate terminal and an emitter terminal.
As shown in FIG. 11, a p-type semiconductor layer 50b (and n-type semiconductor layer) of an overvoltage protection diode are arranged on an insulating film 140 formed on a semiconductor substrate 120, and is covered with an insulating film 150. That is, the overvoltage protection diode is interposed between two insulating films 140 and 150.
Typically, in an overvoltage protection diode, the p-type impurity concentration in a p-type semiconductor layer is lower than the n-type impurity concentration in an n-type semiconductor layer. Consequently, the breakdown voltage (Zener voltage) of an overvoltage protection diode is determined according to the position of a high concentration region (concentration peak) of p-type impurity concentration. In a conventional overvoltage protection diode, as shown in FIG. 11, the p-type impurity concentration is the maximum in a boundary region F10 between the p-type semiconductor layer 50b and the insulating film 150. Consequently, the overvoltage protection diode causes Zener breakdown in the boundary region F10.